JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 16, NUMBER 6, 2015
Jaw position uncertainty and adjacent fields in breast
cancer radiotherapy
Emma Hedin,1a Anna Bäck,1,2 Roumiana Chakarova1,2
Department of Radiation Physics,1 Institute of Clinical Sciences, Sahlgrenska Academy,
University of Gothenburg, Gothenburg, Sweden; Department of Medical Physics and
Biomedical Engineering,2 Sahlgrenska University Hospital, Gothenburg, Sweden
[email protected]
Received 2 March, 2015; accepted 9 July, 2015
Locoregional treatment of breast cancer involves adjacent, half blocked fields
matched at isocenter. The objective of this work is to study the dosimetric effects
of the uncertainties in jaw positioning for such a case, and how a treatment planning protocol including adjacent field overlap of 1 mm affects the dose distribution.
A representative treatment plan, involving 6 and 15 photon beams, for a patient
treated at our hospital is chosen. Monte Carlo method (EGSnrc/BEAMnrc) is used
to simulate the treatment. Uncertainties in jaw positioning of ± 1 mm are addressed,
which implies extremes in reality of 2 mm field gap/overlap when planning adjacent fields without overlap and 1 mm gap or 3 mm overlap for a planning protocol
with 1 mm overlap. Dosimetric parameters for PTV, lung and body are analyzed.
Treatment planning protocol with 1 mm overlap of the adjacent fields does not
considerably counteract possible underdosage of the target in the case studied.
PTV-V95% is for example reduced from 95% for perfectly aligned fields to 90%
and 91% for 2 mm and 1 mm gap, respectively. However, the risk of overdosage
in PTV and in healthy soft tissue is increased when following the protocol with
1 mm overlap. A 3 mm overlap compared to 2 mm overlap results in an increase
in maximum dose to PTV, PTV-D2%, from 113% to 121%. V120% for ‘Body-PTV’
is also increased from 5 cm3 to 14 cm3. A treatment planning protocol with 1 mm
overlap does not considerably improve the coverage of PTV in the case of erroneous
jaw positions causing gap between fields, but increases the overdosage in PTV and
doses to healthy tissue, in the case of overlapping fields, for the case investigated.
PACS numbers: 87.55.D-, 87.55.dk, 87.55.Gh, 87.55.K-, 87.56.JKey words: breast cancer, locoregional treatment, adjacent fields, dose dis­tri­
bution, Monte Carlo
I.INTRODUCTION
Adjuvant radiotherapy after breast conserving surgery is used to reduce the risk for local
recurrences. Regional lymph nodes are included (denoted as locoregional treatment) when
lymph nodes show cancer involvement. In the locoregional case the target volume is treated in
two parts, a cranial and a caudal one. The cranial part covers the lymph nodes and consists of
anterior–posterior fields, whereas tangential fields are most commonly used for the caudal part
(i.e., the breast tissue). The isocenter is positioned at the junction between the cranial and the
caudal fields to eliminate beam divergence, which makes the treatment fields strongly asymmetrical (half-blocked fields). This is a well-established technique introduced many years ago,(1)
and is still used, also in combination with advanced respiratory gating systems.(2)
a
Corresponding author: Emma Hedin, Department of Radiation Physics, Gula straket 2B, Sahlgrenska University
Hospital, SE-413 45 Gothenburg, Sweden; phone: +46(0)70-25 29 856; fax: +46(0)31-3421378; email:
[email protected]
240 240
241 Hedin et al.: Dosimetry for adjacent fields241
It is important to carefully align adjacent fields in order to maintain dose homogeneity in
target without increasing dose to healthy tissue. The issue is closely related to the accuracy of the
jaws positioning. Various methods for matchline dosimetry analysis have been documented.(3,4)
Homann et al.(3) reported a matchline dosimetry analysis tool based on irradiation of film in a
phantom. Different field matching configurations for locoregional breast cancer treatment are
studied in a plane in the abutment region and cold spots are detected. Madebo et al.(4) investigated an implementation of EPID (electronic portal imaging device). A method based on
EPID has been developed earlier in our hospital, where images of adjacent fields are analyzed
for particular gantry angle. Jaw positional uncertainty of up to 1 mm has been detected for the
Varian Clinac iX accelerators in our hospital, sometimes systematic shifts that holds during
entire patient courses. The results from such quality control (QC) tests might be desirable to
take into account in the treatment planning routines. For example, by planning adjacent fields
with a certain overlap if the distance between the fields in the QC reveals gap. However, it is
difficult to predict the absorbed dose that will be delivered in the junction region at the stage of
planning since the technical tolerance in jaw position and in collimator rotation causing misalignment of the jaws results in unknown variations of the junction properties in each particular
case. Treatment machines, regarded as identical, may be different in reality and present opposite
behavior. The importance of alignment of adjacent fields grows with the implementation of
more advanced systems for patient positioning and monitoring, when daily setup errors become
smaller and no longer smoothen eventual dose differences between treatment fractions to the
same degree. A field overlap of 1 mm is recommended in the planning protocol in our hospital
to minimize the risk for underdosage in the target volume. This implies that the risk for hot
spots is deliberately increased, since there is always a trade-off between risk for hot spots and
risk for cold spots. At the stage of planning, the 1 mm overlap is introduced and the matchline
dose assumed to be adequate without further analysis (i.e., dosimetric effect of jaw positioning
uncertainty is not evaluated for). The effect of the matching techniques on the dose variations
in the junction region has been investigated in the case of a breast phantom within the large
multicentre program START.(5) Further dosimetric studies involving patient geometries are
needed to quantify the dose inhomogeneity and evaluate clinical aspects related to it.
The objective of this work is to study the influence of the uncertainties in the jaw position
on the dose distribution in the patient geometry of a locoregional breast cancer treatment and,
furthermore, how a treatment planning protocol including field overlap of 1 mm affects the
situation. This case study will contribute to the understanding of the benefits and disadvantages of using 1 mm overlap and if there is a need for further optimization of such a treatment
protocol. The MC method is used to obtain the dose distributions. It is a reference method for
validation of clinical dose calculations in the presence of heterogeneities, in the penumbra and
in the buildup region and allows for a 3D dose evaluation including the use of dose-volume
histogram parameters currently used to specify dose planning criteria. The effect of ± 1 mm
uncertainty in the jaw positioning is investigated by the two extreme situations of gap and overlap of the adjacent fields that may happen in the reality. In particular, these extremes are 2 mm
gap or overlap in the case of a planning protocol without gap or overlap, as well as 1 mm gap
and 3 mm overlap in the case of a planning protocol with 1 mm overlap (used in our hospital
for all locoregional breast cancer treatments).
II. MATERIALS AND METHODS
Photon treatment fields from Varian Clinac iX accelerators (Varian Medical Systems, Palo Alto,
CA) are considered. A MC model developed earlier,(6,7) built within EGSnrc/BEAMnrc code
package,(8,9) is used for the calculations. The model is expanded for this study by including
multileaf collimator (MLC) and dynamic wedges, as well as correction for backscatter to the
monitor chamber, as described in the Appendix.
Journal of Applied Clinical Medical Physics, Vol. 16, No. 6, 2015
242 Hedin et al.: Dosimetry for adjacent fields242
The capability of the 6 MV model to correctly reproduce asymmetric adjacent fields has been
partly evaluated earlier.(7) This evaluation is extended for the half-blocked fields.
The traditional two-steps approach is utilized, where the particles emerging from the accelerator head for a certain planned field are stored in a phase space file, which is further used as
a source for dose calculations in the geometry of interest.
The Monte Carlo method is chosen to obtain results that are not dependent on a particular
dose calculation algorithm currently available in a treatment planning system. However, test
calculations are performed with the dose calculation algorithm currently used at our hospital
for this type of treatment, namely the analytical anisotropic algorithm (AAA) version 10.0.28
implemented in Eclipse (Varian Medical Systems).
A. Validation of MC beam calculations
The patient case selected as relevant to locoregional treatment in our clinic has a plan with six
fields, as described in Table 1 and illustrated in Fig. 1. The Monte Carlo calculation of each
of the four main fields, excluding MLC and wedges, is validated against measurements using
a water box geometry and setting the gantry angles to zero. Furthermore, the combined dose
distribution from the two main anterior fields (1 and 4) and the two posterior fields (2 and 5),
respectively, is analyzed without MLC and wedges. The dose level for the fields, separate
Table 1. Patient plan details for fields 1 to 6. Fields 1, 2 and 3 are anterior–posterior fields applied to the cranial part
of the target (lymph nodes) for planning protocol with 1 mm overlap (Y1 = 0.1 cm). Fields 4, 5, and 6 are tangential
fields covering the caudal part of the target (i.e., the breast tissue). All fields involve MLC.
Lower (X) Jaw
Upper (Y)
Position
Position
(cm)
Field
Energy
Gantry Angle(cm)
Number
(MV)
(°)
X1
X2 Y1 Y2 WedgeMU
1
2
3
4
5
6
6
15
15
6
6
6
10
183
183
50
229
229
5.5
5.7
5.7
5.5
3.5
5.5
11.50.5
0.511.5
0.56.9
0.1 5 No145
0.1
5.3
20°
70
0.1
5.3
No
11
20 0 No114
19.5
0
15°
106
14.7 0 No11
Fig. 1. Illustration of the patient specific plan. Field numbers indicated in the figure. Fields 1 to 3 cover the cranial part
of the target (lymph nodes); fields 4 to 6 are tangential fields covering the residual breast tissue.
Journal of Applied Clinical Medical Physics, Vol. 16, No. 6, 2015
243 Hedin et al.: Dosimetry for adjacent fields243
and combined anterior/posterior, is verified by ion chamber measurements, (0.125 cm3 PTW
Semiflex chamber 31010; Freiburg, Germany), centrally in the field and in the tail region just
outside the field at 3 cm depth in solid water. The shape of the separate field profiles is verified
with the ion chamber profiler device IC Profiler (Sun Nuclear Corp., Melbourne, FL).
Monte Carlo calculations for the tangential posterior field 5 is also verified with wedge
included using IC Profiler, as well as one ion chamber measurement centrally in the field. The
special consideration of field 5 is motivated by its asymmetry and length in combination with
the wedge.
B. Dose distribution in patient geometry
The dose distribution for the selected treatment plan is calculated with 0.15 × 0.15 × 0.15 cm
resolution in the patient CT images from a Toshiba Aquilion LB CT (120 kV) (Tokyo, Japan).
Tissue segmentation is performed by the formalism in Schneider et al.,(10) as reported earlier.(6)
Clinical target volume (CTV) and planning target volume (PTV) are delineated according to
clinical routine at our hospital.
The MC data are not converted to dose to water, wherefore dose to tissue is reported. The
3D dose distributions are imported as DICOM dose files via Vega library(11) in the Eclipse v.
11.0 (Varian Medical Systems) TPS for viewing and DVH analysis, as well as acquiring of the
dose-volume-parameters such as D98% and V95%.
Dose distributions in the patient CT images are obtained for the following five cases of junction between the cranial fields and the tangential fields: 2 and 1 mm gap, a perfect match, as well
as 2 and 3 mm overlap. The gap and overlap cases are simulated by shortening or extending
the cranial fields. The dose-volume parameters are evaluated for all cases.
Calculations are carried out on a Linux cluster at the National Supercomputer Centre
Linköping, Sweden. The statistical uncertainty (one standard deviation (SD)) of the reported
MC dose is about 2% in the target region.
III.RESULTS
A. Validation of MC beam calculations
The difference between the ionization chamber measurements centrally in the field and MC
data is 0.2%–0.9 % for the four separate main fields (MLC and wedges excluded) where the
uncertainty of the MC data is negligible and the measurement error (95% significance level)
is estimated to below 0.2% for all fields. The comparison between IC Profiler measurement
and MC calculation is shown in Fig. A.1 in the Appendix, Ion chamber measurements are also
shown in Fig. A.1. The agreement between MC and ionization chamber measurements for the
combined anterior/posterior fields is evaluated in the same points as for the separate fields
(i.e., two points in each summed dose distribution) and is within 1%.
An example of the validation of the wedge field calculations is shown in Figure A.2(b) in
the Appendix, where IC Profiler measurement are compared to MC data for the tangential
posterior field, 5, including 15° wedge. The agreement between MC and measurement is for
this case 0.9% centrally in the field.
The MLC component of the MC model is also found to produce results in good agreement
with measurements. The results are not shown here since the MLC does not define the field
edges in the junction region.
B. Dose distribution in patient geometry
The underdosage (2 mm gap) and overdosage (3 mm overlap) in the target volume is illustrated
in the dose-volume histogram (DVH) (Fig. 2). Plan evaluation parameters for PTV, body, and
PTV-body are listed in Table 2. D98% and D2% (near minimum and near maximum dose according to ICRU report 83(12)) are presented in the table to avoid point doses. PTV is 507.6 cm3.
Journal of Applied Clinical Medical Physics, Vol. 16, No. 6, 2015
244 Hedin et al.: Dosimetry for adjacent fields244
Fig. 2. Illustration of the dose distributions in a plane 3 cm dorsal from isocenter for the situation of a) fields overlapping
3 mm and b) with a 2 mm gap. Color scale in Gy per fraction ranging from 0.2 to 3.16. DVH (c) for the case of 3 mm
overlapping fields (solid line) and 2 mm gap (dashed line) between the fields covering the lymph nodes and the tangential
fields irradiating the breast tissue. Perfectly aligned jaws shown for reference (red line).
To further quantify the increased dose in the junction region in the case of field overlaps, the
maximum width in craniocaudal direction of the volume covered by 110% isodose is estimated.
The values obtained are 1.5 cm and 2.1 cm for 2 mm overlap and 3 mm overlap, respectively.
The width of the volume covered by 120% isodose is 0.4 cm and 0.6 cm for 2 mm and 3 mm
overlap, respectively. One hundred and ten percent (110%) and 120% isodoses are not observed
in the case of perfect alignment of jaws.
The impact of jaw positioning errors on the lung dose is mainly due to the uncertainties of the
cranial fields (when extended in caudal direction more lung is in-field). Relevant subregion can
be analyzed to further quantify the local effects that are barely seen in the DVHs. A subvolume
of 145 cm3 lung tissue around the junction region with 3 cm width in craniocaudal direction is
considered. The mean dose changes from 23.8 Gy for perfectly aligned jaws (prescribed dose
to the tumor is 50 Gy in 25 fractions) to 26.3 Gy for a 2 mm jaw positioning change (1 mm
extension of field according to protocol and 1 mm uncertainty) — an increase of 2.5 Gy.
Furthermore, the results in this study indicate hot spots in the lung tissue close to the chest
wall in the case of field overlap. The dose level and volume of those hot spots are difficult to
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245 Hedin et al.: Dosimetry for adjacent fields245
Table 2. Plan evaluation measures for PTV, body, and body-PTV.
PTV
Body
Jaws
2 mm
Apart
Jaws
Jaws
1 mm
Perfectly
Apart
Aligned
V95% (%)
90
91
V105% (%)
16
16
V110% (%)
0.2
0.3
V120% (%)0.0 0.0
D2% (%)
108
108
D98% (%)85 88
Dmean (%)
101
101
V105% (cm3)
207
216
V110% (cm3)
13
13
V120% (cm3)0 0
V95% (cm3)
503
V105% (cm3)
126
Body-PTV
V110% (cm3)
11
V120% (cm3)0
510
129
11
0
Fields
Overlapping
2 mm
Fields
Overlapping
3 mm
94
17
0.4
0.0
109
91
101
95
22
3.0
1.4
113
92
102
95
23
4.0
2.1
121
92
103
224
15
0
268
47
15
283
59
25
526
133
12
0
547
153
31
5
558
164
37
14
V95% (%) = 90 means that 90% of the organ volume received 95% of the prescribed dose or more; D2% (%) = 108
means that 2% of the organ volume received 108% of prescribed dose or more.
estimate because of interface uncertainties in the MC tissue segmentation procedure involving voxel interpretation of the CT scan. The lung tissue (defined by the clinical segmentation
wizard) in the subvolume considered above has an increase in D2% from 44.5 Gy (perfectly
aligned jaws) to 45.4 Gy for a 2 mm overlap. The moderate increase in D2% indicates that the
volume of the hot spots is small.
IV.DISCUSSION
When gap is present, the largest concern is to evaluate possible cold spots in the target volume.
The D98% (near minimum dose) in the PTV is reduced from 91% for perfectly aligned fields
to 88% and 85% for a 1 mm and 2 mm gap, respectively (see Table 2). The target coverage
expressed as the PTV volume covered by the 95% isodose, V95%, is reduced from 94% to 91%
and 90% respectively for a 1 and 2 mm gap. Thus, for 95% isodose coverage there is not a
large distinction between gaps of 1 or 2 mm. When overlap is present, the PTV volume covered
by 105% and 110% isodoses increases. A volume covered by 120% isodose appears, as well.
However, when comparing the two cases of overlap, the largest effect is seen for D2% (near
maximum dose). This is to be expected, since the effect of overlapping fields is restricted to a
small part of the dose distribution. In the clinical evaluation of a treatment plan, the risk for hot
spots in target may not be the largest concern, but rather the risk for hot spots in normal tissue.
When overlap is present, even the volume outside target (Body – PTV in Table 2) covered
by 110% isodose increases, from 12 cm3 to 31 cm3 and 37 cm3 for 2 and 3 mm overlap. Also, a
region of 15 cm3 confined by 120% isodose appears for 2 mm overlap and increases to 25 cm3
for 3 mm overlap. The region exposed by 110% dose or more does not include lung tissue, but
other organs at risk, such as the plexus brachialis, may be present in this region. The dimensions
of the 110% region in the craniocaudal direction are about 1.5–2 cm larger than in the case of
perfectly aligned jaws. Thus, the everyday setup uncertainty we observe at our hospital cannot
fully smoothen the effect.
The changes in mean dose, V20Gy and D2% for the ipsilateral lung are small due to a large
organ volume. However, the analysis of the 3 cm wide subvolume in the lung around the junction
reveals larger changes, as pointed out in Results section B. Thus, analysis of small regions may
be more appropriate to detect local dose changes than investigation of the dose distribution in
Journal of Applied Clinical Medical Physics, Vol. 16, No. 6, 2015
246 Hedin et al.: Dosimetry for adjacent fields246
whole lung. It should be stressed, that the definition of the subvolume is not based on anatomical
features and the dose evaluation parameters have no clear clinical meaning.
Analysis of the test calculations by AAA reveals a qualitative agreement with the MC results
and conclusions. An example of the dose levels predicted in soft tissue in the junction region
by AAA and MC, respectively, is shown in Fig. A.3 in the Appendix. Larger differences are
observed between MC and AAA for profiles involving lung. More detailed quantitative comparison between AAA and MC data would require thorough investigation of AAA performance
in penumbra regions and interfaces between soft tissue, bone, and lung tissue. The investigation
of the impact of choice of algorithm in the clinical treatment planning system is important, but
beyond the scope of this study.
The two treatment planning protocols, namely, planning without overlap of adjacent fields
and planning with 1 mm overlap, can be discussed on the basis of the dose distribution analysis.
For planning without overlap, the extreme cases are 2 mm gap and 2 mm overlap, respectively.
The risk for insufficient coverage of PTV (e.g., decrease of V95% by 4%) should be balanced
with the risk for increased dose to healthy tissues (e.g., 110% dose to 47 cm3 soft tissue) and
increased local dose to the ipsilateral lung compared to the reference case with a perfect field
alignment. For planning with 1 mm overlap, the extreme cases are to have 1 mm gap and 3 mm
overlap of the adjacent fields. This strategy is used to secure the PTV coverage, but will entail
an increased risk for higher doses to healthy tissues. Since V95% is similar for both cases of
gap, the advantage of the planning protocol with 1 mm overlap over this without overlap is
not clearly seen. The risk for increased dose to soft tissue and lung are seen to be higher for
3 mm than for 2 mm overlap (and definitely higher than for perfectly aligned jaws). However,
more detailed knowledge is needed on the risk for recurrence in the junction region and the
clinical significance of the local increase of the lung dose before the effects can be properly
evaluated. Risk factors like radiation induced brachial plexopathy, for example, should be
taken into account, as well, and a maximum dose of 54 Gy (or lower to take into account the
risk of increased dose due to uncertainties in jaw positioning) to plexus brachialis should be
added to the analysis.(13,14) Elaboration on specific clinical recommendations might require
different approaches, depending on treatment technique, diagnosis, and other patient specific
circumstances, which is outside the scope of this study.
The results from this case study indicate that the use of a planning protocol with 1 mm overlap can be debated. Further studies on more patients are valuable to establish the dominantly
negative effect found in this study of using a treatment protocol based on 1 mm field overlap.
In general, the policy of the treatment planning protocol depends on the rules for target
delineation. According to the Swedish national guidelines, the remaining breast parenchyma and
ipsilateral regional lymph nodes in the axillary level III and supraclavicular fossa are included
in one CTV and, consequently, comprise one PTV. An alternative approach to target volume
delineation is to consider the residual breast tissue and the lymph nodes as separate CTVs
and consequently PTVs.(14) In this way, different constrains can be defined for each PTV. In
the work cited above, it is concluded that gaps between adjacent lymph node volumes should
be avoided and the importance of a homogeneous dose in the intersection between different
lymph node targets is stressed. Gaps between the residual breast tissue and the lymph nodes
are not discussed. An interface between the residual breast tissue PTV and lymph node PTVs
may allow larger flexibility in the planning stage.
Achieving a good PTV coverage has high priority in the treatment planning procedure. It is
important to avoid gaps and, therefore, a planning protocol with 1 mm overlap was the choice at
our hospital (i.e., for all locoregional breast cancer treatments). The results from this case study
promote and facilitate a discussion on how the overlap can be adjusted for different groups of
patients stratified, for example, according to stage of cancer. For the different groups of patients,
avoiding underdosage of PTV and reducing dose to healthy tissues may have different priorities.
A variable placement of the junction between the adjacent fields may be implemented to
smooth out the dose inhomogeneity in the junction region. The implementation may raise ­practical
Journal of Applied Clinical Medical Physics, Vol. 16, No. 6, 2015
247 Hedin et al.: Dosimetry for adjacent fields247
issues of having two active plans for a patient. Also the effects of the position of the junction
on the dose coverage should be considered. This case is not investigated in the current work.
As the patient positioning techniques are improved by for instance daily imaging and surface
scanning the setup errors are expected to decrease and no longer smoothen the effect of field
gaps/overlaps to the same degree. Furthermore, less smearing due to setup errors is also true
for the case of hypofractionation. Hypofractionation also means that the biological effect of
hot spots will be larger.
V.CONCLUSIONS
A treatment planning protocol with 1 mm overlap does not considerably improve the coverage of
PTV in the case of erroneous jaw positions causing gap between fields, but increases the overdosage
in PTV and the dose to healthy tissue, in the case of overlapping fields, for the case investigated.
Therefore, a treatment planning protocol including 1 mm field overlap can be questioned. Before
recommendations are made further investigations are needed, which should consider, for example,
decreased daily setup errors, hypofractionation, and negative side effects in healthy tissue.
ACKNOWLEDGMENTS
This study was supported by grants from the King Gustav V Jubilee Clinic Cancer Research
Foundation, Lions Cancer Research Foundation, Assar Gabrielsson Research Foundation, and
Percy Falk Research Foundation.
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248 Hedin et al.: Dosimetry for adjacent fields248
APPENDICES
Appendix A: The Monte Carlo Model
A. Monte Carlo model, absolute dose calculations
The formalism for conversion of the MC dose in Gy per primary history to the dose in Gy for
a certain number of monitor units MU (denoted further in the text as absolute dose) is based
on simulations of the calibration geometry and corrections for the effect of backscattered
radiation to the monitor chamber, as described in Popescue et al.(A1) The accelerators in our
clinic are calibrated in water at 10 cm depth at source-to-surface distance (SSD) 90 cm for a
10 cm × 10 cm field. A backscatter correction factor (BSCF) is used that relates the amount
of backscattered dose for a certain field to the calibration field size. A linear dependence is
considered between the backscattered dose to the monitor chamber and the field size as suggested
by Verhaegen et al.(A2) It is assumed that the effect of the components located below the upper
Y jaw, namely the lower X jaw and the MLC, is negligible. This assumption is consistent with
the results reported on the dominating effect of the upper Y jaw on the backscatter compared
to that of the lower X jaw.(A2,A3) The BSCF is therefore only dependent on the field length in
the Y direction (FSy) and is given by:
BSCF(FSy) =
a+b*10
a+b*FSy
(A1)
New parameter values of a and b in Eq. (A1), specific for our accelerators, are obtained,
namely, a = 1.034 (1.028) and b = -0.00085 (-0.00070) for 6 (15) MV, respectively. The field
sizes included in this optimization procedure are 4 × 4 cm, 20 × 20 cm, and 40 × 40 cm symmetrical square fields, as well as 4 × 20 cm and 20 × 4 cm symmetrical rectangular fields.
Wedge fields are generated by the DYNJAWS(9,A4) code option following Varian Enhanced
Dynamic Wedge (EDW) implementation. The dynamic movement of the upper jaws is controlled by the so-called segmented treatment tables, STT. Each STT contains information on
the jaw position versus dose delivery information at different instances of the EDW field in
form of cumulative weighting of monitor units (MU). A single STT, (the one for 60° wedge),
is used to generate all the other STTs for various field sizes and wedge angles. The input file
for DYNJAWS is generated by the AUTOJAWS script.(A5)
For wedges, the backscatter correction is applied on the differential segmented treatment
table; STTdiff,i = STTi – STTi-1, where i is an index indicating the row of the STT. To facilitate
the writing in Eq. (A2), it is defined that STT0 = 0. The row-index, i, varies from 1 to maximum
number of rows in the STT. Each row of the backscatter corrected STT, STTbscorr, is thereby
given by:
STTbscorr,i = Σ1i ((STTi – STTi–1) * BSCF(FSyi))(A2)
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249 Hedin et al.: Dosimetry for adjacent fields249
In this way, the backscatter effect is taken into account when simulating the jaw movement.
The backscatter corrected STT is normalized to the number of cumulative monitor units,
delivered at the last position of the jaw, before it is used in the EGSnrc/BEAMnrc for producing a phase space. The number of MUs of a wedged field is in the treatment plan equal to the
cumulative number of MUs delivered at the last position of the jaw. Therefore, a backscatter
correction factor (denoted global in the text) is needed also for wedged fields so that the total
number of MUs can be corrected in a similar way as for to the nonwedge fields. This global
backscatter correction factor used for wedge fields in the conversion of the MC dose to absolute
dose is obtained by the ratio between the cumulative number of MUs for backscatter corrected
and non-corrected STT, respectively.
B. Monte Carlo model, validation
Validation results for the four main fields, no wedges included, are presented in Fig. A.1.
The calculated anterior cranial and tangential fields 1 and 4, ((a) and (b) in Fig. A.1), and the
posterior cranial and tangential fields, 2 and 5, ((c) and (d) in Fig. A.1), are compared with
IC Profiler and point measurements by ionization chamber at 3 cm depth in solid water. The
absolute dose (Gy for the specified number of MUs for each particular field) is given to test the
validity of the dose conversion procedure. The agreement between MC and ionization chamber
measurements in the central parts of the fields is within 1%. The MC calculations in the penumbra and the tail regions comply very well with the IC Profiler measurements as well as seen
in the figure.
MC simulated wedge fields are validated by relative comparison of MC and measured dose
profiles in solid water by IC Profiler (Sun Nuclear Corp.) containing 251 ion chambers with
2.9 mm width and 5 mm spacing, as well as by absolute dose measurements with ionization
chamber CC13 (IBA Dosimetry, Schwarzenbruck, Germany). The validation of the backscatter
correction for the wedged fields is shown for one of the patient treatment fields in Fig. A.2. The
special consideration of this field is motivated by its asymmetry and length in combination with
the wedge. Part of the MUs are delivered when jaw opening is exceptionally far away from the
central axis (i.e., 19.5 cm), as compared to the small wedged posterior fields of 15 MV photons
irradiating the cranial part of the target.
For simulated and measured wedge profiles for 6 MV and 15 MV, symmetric 20 × 20 cm
field with 45° wedge an agreement within 1.6% is observed, except in the horns where deviations up to 3.6% are detected. For the MLC validation, the agreement between theoretical and
experimental data is within 1.5%, except for the tails where the deviation is 2.5%.
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250 Hedin et al.: Dosimetry for adjacent fields250
Fig. A.1. Dose profiles in water at 3 cm depth for the main fields of the validation plan: (a) tangential anterior field;
(b) lymph nodes anterior field; (c) tangential posterior field; (d) lymph nodes posterior field. Black lines = MC calculations, gray dots = IC Profiler measurements, large crosses = ionization chamber measurements.
Fig. A.2. The STT for the wedge tangential posterior field (a). Backscatter corrected (dashed) shown together with the
original uncorrected STT (solid) given by the manufacturer for wedge angle 15°. Validation profile (b) at 3 cm depth in
water for the tangential wedge field (6 MV). MC calculations (solid black line) compared to IC Profiler measurement
(grey dots) — a relative measurement normalized to the absolute ion chamber measurement (cross).
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251 Hedin et al.: Dosimetry for adjacent fields251
C. Dose profiles in junction region
An example of the dose levels predicted in the junction region by AAA and MC, respectively,
is shown in Fig. A.3.
Fig. A.3. To the left the dose distribution for 3 mm overlapping fields is shown where the black line indicates the position
of the dose profile shown in the graph to the right. MC dose profiles for 2 mm gap and 3 mm overlap (green and yellow)
compared to AAA dose profiles (blue and red) for the same cases.
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